EP0667200B1 - Preparation of high-purity metal powder from metal alkoxides - Google Patents

Abstract

Prodn. of high purity metal powder is effected by reaction of volatile alkoxide cpd(s). with a redn. gas, pref. H2 opt. diluted with an inert carrier gas selected from the noble gases, pref. Ar. Pref. the metal alkoxide is an alkoxide of a gp. 3-14 element (IUPAC 1985) and reaction is carried out at 400-1400 (pref. 600-1200) degrees C. The alkoxide is pref. selected from chromium tert.-butoxide, niobium methoxide, niobium ethoxide, tantalum methoxide, tantalum ethoxide, tungsten methoxide and tungsten ethoxide.

Description

The present invention relates to a method for producing high-purity Metal powder.

The microfabrication of highly integrated electronic components always poses higher requirements for the purity of the interconnect metals, e.g. Titanium, Niobium, tantalum, molybdenum or tungsten. Especially the radioactive elements Thorium and uranium, as α-emitters, can have serious consequences in highly integrated applications Cause memory chips.

In Semiconductor Materials and Process Technology Handbook for Very Large Scale Integration (VLSI) and Ultra large Scale Integration (ULSI), Gary E. McGuire, Editor, Noyes Publications, pages 575-609 and in Silicon Processing for the VLSI Era, Lattice Press, pages 384-406 is an overview of the conventional requirements regarding electrical conductivity and temperature resistance of interconnect metals. Because with increasing Integration density not only the number of connections required, but also the average length of the interconnects between the active switching elements increased, purity requirements for interconnect metals are increasing, which are usually applied by sputtering or evaporation, posed.

According to N.N. Greenwood and A. Earnshaw, Chemistry of the Elements, Pergamon Press, 1984, page 1113 is the process for the production of high purity titanium van Arkel and de Boer known. In this process, what is to be cleaned Raw titanium together with iodine in an evacuated vessel heated to ∼500 ° C, whereby gaseous titanium iodide forms, which in turn forms on a Tungsten wire electrically heated to C1200 ° C elsewhere in the Equipment decomposed to high-purity titanium. A disadvantage of the method is that only small quantities can be produced in this way and a A number of other elements such as Zirconium, hafnium and above all Thorium can also be transferred.

According to the in the Kirk-Othmer Encyclopedia of Chemical Technology, Volume 22, Third edition, pages 541-564 described prior art for manufacturing of tantalum metal come as alternative methods for the production of pure metal Purification by fractional crystallization and purification by Liquid phase extraction in question. The principle of liquid phase extraction is based the different solubility of the metal fluorides in a two-phase system from dilute acid and an organic phase, e.g. Methyl isobutyl ketone. The separation of tantalum and niobium is disclosed in US Pat. No. 3,117,833 described this way.

Separation and purification of the desired metal species can also be done via Ion exchange resins are used, as in Metallurgy of the Rarer Metals, Volume 6, Tantalum and Niobium, page 129-133.

Distillative separation via the metal halides, e.g. Tungsten hexafluoride in principle also possible. This method is the subject of JP-A 02 30,706. Tungsten hexafluoride is converted to tungsten powder with hydrogen at 650-1400 ° C reduced, which is suitable for the production of sputtering targets. Disadvantage of this The process is that in the reduction with hydrogen, a large amount of hydrogen fluoride attack.

The object of this invention was therefore to provide a process for the production of to provide high purity metal powder which is simple and can be carried out inexpensively. This problem is solved by Implementation of volatile, i.e. sublimable or distillable, Metal alkoxides with a reaction gas.

This method is the subject of this invention.

The metal alkoxide compounds which are used according to the invention have the general formula M (OR) _x , where M is a metal from Group 3-14 (according to IUPAC 1985), R is an alkyl, aryl, cycloalkyl or aralkyl radical and M (OR ) _{x is} a sublimable or distillable compound. In the table below, some alkoxide compounds that are suitable according to the invention are listed as examples. Metal alkoxide boiling point Aluminum isopropylate 128 ° C / 5 mbar Chromium (IV) tert-butoxide 66 ° C / 3.6 mbar Gallium ethylate 185 ° C / 0.7 mbar Niobium methylate 153 ° C / 0.13 mbar Niobium ethylate 156 ° C / 0.07 mbar Tantalum methylate 130 ° C / 0.3 mbar Tantalum ethylate 146 ° C / 0.2 mbar Titanium ethylate 104 ° C / 1.3 mbar Tungsten methylate 90 ° C / 0.5 mbar

Chromium tert-butoxide, niobium methoxide, Niobium ethoxide, tantalum methoxide, tantalum ethoxide, tungsten methoxide and tungsten methoxide

The reaction gas in the reaction according to the invention is preferably hydrogen. The reaction gas can also be mixed with an inert carrier gas, in particular argon be diluted.

The method according to the invention is preferably at a temperature between 400 ° C and 1400 ° C carried out. The most preferred reaction temperature lies between 600 ° C and 1200 ° C.

To produce high-purity metal powder, it is expedient to use the metal alkoxide by distillation or sublimation in a PVDF apparatus and then reduce in hydrogen flow. This way too the contaminants that arise from handling in glass equipment, such as e.g. Aluminum, calcium, magnesium and silicon led to below 0.5 ppm will.

For the production of the metal alkoxides it should be noted that the conventional Process of alkoxide synthesis from metal chloride and alcohol in the presence of a Base as e.g. for the production of tantalum alkoxides in J. Chem. Soc. 1955, Page 726-728 is always leads to compounds containing chloride. Other alkoxides such as the tungsten alkoxides are cooked in this way inaccessible.

After Z. Anorg. Chem. 1932, 206, 423, the conventional method for alkoxide synthesis from metal chloride and alcohol in the presence of ammonia is unsuitable for tungsten (VI) alkoxides, because WCl ₆ reacts directly with ammonia to form a tungsten nitride.

According to Angew. Chem. Int. Ed. Engl. 1982, 94, 146-147, WF _{6 is} converted into W (OCH ₃ ) ₆ in an equilibrium reaction with volatile Si (OCH ₃ ) ₄ as ligand transfer agent. Complete methoxylation is only possible by treating the partially fluorinated product with a methanolic NaOCH ₃ solution.

From Inorg. Chem. 1977, 16, 1794-1801 it is known that tungsten (VI) alkoxides out the reaction of tungsten (VI) hexakis (dimethylamide) and the corresponding Alcohol can be made. However, the synthesis of the tungsten amide compound according to Inorg. Chem. 1977, 16, 1791-1794 very complex and is therefore ruled out for a large-scale process.

In our experience, electrochemical processes according to US Pat Zhurnal Obshchei Khimii) 1985, 55, 2130-2131. In these processes, an anode made of tungsten is dissolved by anodic oxidation in an alcoholic electrolyte solution according to reaction equation (1). W + 6 ROH → W (OR) ₆ + 3 H ₂

Suitable reactors for carrying out the process according to the invention can Furnaces with an adjustable atmosphere or gas phase reactors. Since the metal alkoxide compounds according to the invention all in a simple manner in the gas phase to be brought is also a gas phase reactor according to DE-A 4 214 720 suitable. The choice of reactor is determined by the particular requirements regarding fine particle size and particle size distribution of the metal powder.

The present invention will be described in more detail below with the aid of a few examples are explained without restrictions for obvious variations of the Litigation. First, the synthesis of some tungsten alkoxides that are used Suitable for carrying out the present invention, described (preliminary experiment 1 and 2).

Preliminary test 1 Electrochemical production of tungsten (VI) methoxide

A 0.5 molar solution of LiCl in methanol was electrolysed under argon protective gas in a flat ground reaction vessel which was provided with a steel cathode, an anode made of tungsten and a reflux condenser. It was electrolyzed with direct current and a current density of 200 mA / cm ² . The electrolyte solution turned yellow-orange and began to boil shortly after the start of electrolysis.

After the electrolysis, the excess methanol was removed in vacuo at room temperature. The dry residue was taken up in hexane, briefly brought to the boil under reflux and separated from the undissolved portion by means of a reverse frit. The filtrate is distilled. After removal of the hexane, W (OCH ₃ ) ₆ boils at ∼90 ° C / 0.5 mbar. The compound is colorless and solidifies at 50 ° C. Elemental analysis: W, found 48.3%, calculated 49.7%; C, found 19.6%, calculated 19.5%; H, found 4.7%, calculated 4.9%; Cl found 22 ppm.

Preliminary test 2 Electrochemical production of tantalum methoxide

A solution of 50 g of NH ₄ Cl in 2000 ml of methanol was electrolysed under argon protective gas in a flat-ended reaction vessel which was provided with a steel cathode, an anode made of tantalum and a reflux condenser. It was electrolyzed with direct current and a current density of 200 mA / cm ² . The electrolytic solution turned yellowish and began to boil shortly after the start of the electrolysis.

After the electrolysis, the excess methanol was removed in vacuo at room temperature. The dry residue was taken up in hexane, briefly brought to the boil under reflux and separated from the undissolved portion by means of a reverse frit. The filtrate is distilled. After removal of the hexane, Ta (OCH ₃ ) ₅ boils at ∼130 ° C and 0.3 mbar in vacuum. The compound is colorless and solidifies at ∼50 ° C. Elemental analysis: Ta, found 50.2%, calculated 53.8%; C, found 17.9%, calculated 17.9%; H, found 4.6%, calculated 4.5%; Cl found 19 ppm.

example 1 Manufacture of tungsten powder

Electrochemically produced tungsten ethoxide is purified by sublimation in a glass apparatus and then reacted with hydrogen in a tube furnace at 1000 ° C., equation (2). W (OCH ₃ ) ₆ + 3 H ₂ → W + 6 CH ₃ OH

The tungsten metal powder was analyzed for impurities using GDMS (glow-discharge mass spectroscopy). Analysis of the tungsten metal powder, values in ppm. Al 1 B <0.05 Ba 0.09 Bi <0.02 Approx 0.34 CD <0.05 Co 0.08 Cr 0.26 Cu 0.06 Fe 0.31 K <0.05 Mg 5 Mn 0.015 Mon 6 N / A 0.2 Ni 0.12 P 0.19 Pb 0.03 Sb <0.05 Si 9 Sn <0.05 Sr. <0.02 Th <0.0005 Ti 0.48 U <0.0005 V <0.02 Zn <0.02 Zr <0.05

Example 2 Production of tantalum powder

Electrochemically produced tantalum methoxide is purified by distillation at 130 ° C. in a vacuum (0.3 mbar) in a glass apparatus and then reacted with hydrogen in a tube furnace at 1000 ° C., equation (3). Ta (OCH ₃ ) ₅ + 2½ H ₂ → Ta + 5 CH ₃ OH The tantalum metal powder was analyzed for impurities using GDMS (glow-discharge mass spectroscopy). Analysis of the tantalum metal powder, values in ppm. Al 0.5 B <0.05 Ba 0.09 Bi <0.02 Approx 0.4 CD <0.05 Co 0.05 Cr 0.04 Cu 0.06 Fe 0.2 K <0.05 Mg 3rd Mn 0.015 Mon 0.9 N / A 0.4 Nb 8th Ni 0.15 P 0.1 Pb 0.03 Sb <0.05 Si 7 Sn <0.05 Sr. <0.02 Th <0.0005 Ti 0.6 U <0.0005 V <0.02 Zn <0.02 Zr <0.05

Example 3 Manufacture of titanium powder

Electrochemically produced titanium ethoxide is purified by distillation at 104 ° C. in a vacuum (0.3 mbar) in a glass apparatus and then reacted with hydrogen in a tube furnace at 1000 ° C., equation (4). Ti (OC ₂ H ₅ ) ₄ + 2 H ₂ → Ti + 4 CH ₃ OH The titanium metal powder was analyzed for impurities using GDMS (glow-discharge mass sprectroscopy). Analysis of the titanium metal powder, values in ppm. Al 2nd B <0.05 Ba 0.5 Bi <0.02 Approx 0.2 CD <0.05 Co 0.25 Cr 0.15 Cu 0.06 Fe 0.4 K <0.05 Mg 3rd Mn 0.01 Mon 0.4 N / A 0.3 Nb 0.25 Ni 0.15 P 0.2 Pb 0.02 Sb <0.05 Si 6.5 Sn <0.05 Sr. <0.02 Th <0.000 5 U <0.0005 V <0.02 Zn <0.02 Zr 6

Claims (8)

1. Method for the preparation of high-purity metal powder, characterised in that the preparation is carried out by the reaction of one or more volatile metal alkoxide compounds with a reducing gas.
2. Method according to claim 1, characterized in that hydrogen is used as the reducing gas.
3. Method according to one of claims 1 or 2, characterised in that the reducing gas is diluted with an inert carrier gas from among the rare gases.
4. Method according to claim 3, characterised in that the carrier gas is argon.
5. Method according to one or more of claims 1 to 4, characterized in that the metal alkoxide is an alkoxide of an element of the groups 3 to 14 of the periodic system of elements.
6. Method according to one or more of claims 1 to 5, characterized in that the alkoxide compounds are preferably methoxides.
7. Method according to one or more of claims 1 to 6, characterized in that the alkoxide compound is tungsten methoxide and/or tantalum methoxide.
8. Method according to one or more of claims 1 to 7, characterized in that the reaction is carried out at a temperature between 400°C and 1400°C and particularly preferably between 600°C and 1200°C.